[Sweephand]

It could indeed be stone / marble - Citizen used some unusual case materials in the early 1970's, including stone and wood:

Here's an example of one with a kind of agate case: http://citizen.jp/discovery/model/1970/dresswatch.html

This one has stone case: http://seikoholics.yuku.com/sreply/472

And this one is wood, made from specially treated birch (these were known as 'Afrocraft'), rose wood was also used : http://citizen.jp/discovery/model/1970/dresswatch.html

Obsidian and Tiger Eye stone was also used. Not seen one like yours before though.

Is there a case material code on the back? May be the lettering above the Citizen logo, but I can't make it out.

Interesting piece

Sweephand

 

[Sweephand]

'UHC' is the case material code, and is not one I've seen before. This fits with the fact that this is an unusual case and is likely therefore to be authentic and original.

6501 is of course the caliber number - the 6xxx series were among the last of Citizen's automatic movements before the quartz revolution, along with the 8xxx series which included the 8100 and 8110 chronographs. The 6501 was first produced in 1971, but its production run wasn't very long, with Citizen concentrating on the 8200 as its workhorse movement (like the Seiko 7s26) from the mid-70's onwards.

4-652797TA is the case code, which appears to incorporate reference to the movement in this model via the first two digits - that isn't always so with Citizens unfortunately so there doesn't appear to be any obvious method in case codes.

40400887 is the serial number - as we know this movement was produced in the 1970's, this means that the year of production was 1974 (first digit), and the month was April (second & third digits, '04').

61-0992 is the model number, specific to this design. This kind of number was used in the later vintage watches, but was not applied nearly so widely as the Seiko equivalents.

GN-4-S is the case construction code, to assist servicing and repair. There were a number of 'GN' types, but I think they were of the screw type like yours.

Sweephand